As to a high power generator such as a boiler, a gas turbine, a jet engine, a diesel engine or the like, there are many studies for mainly improving a heat efficiency. However, the improvement of heat efficiency forces a severe thermal load to a constructional part (material). As a metal material used in a high temperature portion of the high power generator, therefore, it is required to have excellent high-temperature oxidation resistance and high-temperature corrosion resistance. Particularly, if it is intended to use a fuel containing impurities such as V, Na, S and the like, since inorganic compounds containing these impurities violently corrode and wear the metallic material at a high-temperature state, it is necessary to maintain the metallic material at a stable state for a long time under such an environment.
In order to satisfy such a requirement, there have been developed many heat resistant alloys consisting essentially of nonferrous metallic elements such as Cr, Ni, Mo, Co, W, Ta, Al, Ti and the like from the old time, which are called as so-called super-alloys.
In these heat resistant alloys, however, the high-temperature strength is most preferential, so that it tends to suppress an addition amount of a metal element not serving to the improvement of the strength as low as possible. A typical example of the metal element not serving to the improvement of the strength is Cr, Al, Si or the like, but these elements are excellent in the oxidation resistance and resistance to high-temperature corrosion. Therefore, it is ordinary that the super-alloy preferentially requiring the above high temperature strength is poor in the oxidation resistance and resistance to high-temperature corrosion.
Heretofore, a metal elements such as Cr, Al, Si or the like or an alloy thereof has previously been applied to a surface of a super-alloy member used under a high temperature environment through a spraying process, a diffusion treatment method or the like to compensate for the lowering of resistance force to chemical damage of the super-alloy.
In the spraying process, it is merit that the kind of the spraying material may optionally be selected, but there is a drawback that the resulting coating is porous and is poor in the corrosion resistance and adhesion property because the treatment is carried out in air.
In this connection, there has recently been developed a method wherein plasma spraying is carried out in an argon gas atmosphere of a low pressure containing substantially no air (oxygen) (low-pressure plasma spraying process), whereby drawbacks inherent to the atnospheric-sprayed coating have largely been improved, but it can not be said that such a coating is still sufficient under recent environment exposed at higher temperature.
On the other hand, the conventional diffusion treatment method is relatively easy when Cr, Al, Si and the like are treated individually, but it is hardly said that the oxidation resistance and the resistance to high-temperature corrosion are sufficient. And also, this treatment is required to be conducted at a higher temperature state of about 1000.degree. C., so that it has a drawback that mechanical properties of super-alloy matrix lower.
Under the above circumstances, JP-A-55-104471 proposes a method wherein Ni--Cr alloy as an oxidation-resistant metal is sprayed to conduct a diffusion treatment of Al, Cr or the like.
In this method, however, the treatment at high temperature can not be avoided, so that it is inevitable to lower the mechanical properties of the matrix though the adhesion property and denseness of the sprayed coating are improved.
On the other hand, the development of spraying materials is carried out for using under high-temperature environment. A typical material is a heat-resistant alloy material represented by MCrAlX (wherein M is a metal of Ni, Co or Fe or a mixture thereof. X is an element such as Y, Hf, Ta, Cs, Pt, Ce, Zr, La, Si, Th or the like). By plasma spraying the MCrAlX alloy under a low pressure, it is possible to form a sprayed coating having excellent oxidation resistance and resistance to high-temperature corrosion, and the performances of the high-temperature member are more improved. And also, there is proposed a method wherein the diffusion treatment of Cr, Al or the like is conducted after the formation of the MCrAlX alloy sprayed coating (e.g. JP-B-61-10034).
The service life of the member for gas turbine is fairly improved by the above recent spraying process or the development of a technique combining the spraying process and the diffusion treatment method. However, the gas temperature of the gas turbine is anticipated to reach 1500.about.1700.degree. C. in future.
In the gas turbine member contacting with such a high temperature gas, even if it is reinforced with a cooling system through air or steam, it is anticipated that the temperature of the exposed member exceeds the existing 900.degree. C. and will be 950.about.1050.degree. C. For this end, it is studied to improve the high temperature strength of the member for the gas turbine (metal matrix).
However, when the alloy sprayed coating is formed on such a high temperature member (metal substrate) by the conventional method, there is a fear of causing the following problems.
(1) In the high temperature member (metal substrate), as the temperature becomes high, there is a strong tendency that an alloy component in a sprayed coating of, for example, MCrAlX alloy (wherein M is one or more of Ni, Co and Fe, X is one or more of Y, Rf, Ta, Cs, Pt, Ce, Zr, La, Si and Th) formed on the surface or a composite sprayed coating containing a diffusion layer therein diffuses and penetrates into the inside of the metal substrate and hence a thick brittle layer is created in a boundary portion between the sprayed coating and the substrate surface to easily peel the sprayed coating.
(2) Among the components of the MCrAlX alloy penetrated into the inside of the metal substrate, Al particularly reacts with Ni included in the metal substrate to produce a brittle intermetallic compound such as AlNi, AlCo or the like and has an action disappearing a high temperature strength component or precipitate existing in the substrate. As a result, the high temperature strength of the substrate as a whole lowers and it is apt to cause crack or local breakage resulted from thermal fatigue.
(3) Even in the MCrAlX alloy sprayed coating, Ni, Cr and the like being basic components diffuse to form a brittle layer, so that the resistance to thermal shock in the coating considerably lowers.
It is said that these problems result from the feature that the components of the MCrAlX alloy sprayed coating applied to the surface of the substrate diffuse and penetrate into the substrate. As a countermeasure therefor, it is considered that the prevention of diffusing and penetrating the alloying components of the sprayed coating is an effective means.
As such a means, there is a method wherein a high melting point metal (Nb, Ta) or a thin layer of (10.about.100 .mu.m) of an oxide film such as Al.sub.2 O.sub.3 is directly formed on the surface of the substrate through a spraying method or PVD method and the conventional MCrAlX alloy sprayed coating is formed thereon to control the internal diffusion of the MCrAlX alloying components.
In this method, however, an expensive metal different from the MCrAlX alloying components is used, or the PVD method using an electron beam as a heat source is used in the formation of Al.sub.2 O.sub.3 film, so that the different film-forming process should be adopted and hence the lowering of the productivity is caused.
And also, the prevention of the internal diffusion of the alloying components by this method loses the diffusion layer required for ensuring the adhesion property between the substrate and the sprayed coating and the adhesion property of the coating considerably lowers.
It is, therefore, an object of the invention to solve the aforementioned problems of the conventional techniques and to propose a coating technique for advantageously solving problems such as lowering of productivity, contamination of alloy coating based on the adoption of different metal, cost up based on the adoption of different coating process and the like.